The Oceanic Southern Mode Revisited
Hughes, Chris; Williams, Joanne

It is now well established that, on timescales shorter than about a year, variations in Antarctic circumpolar ocean transport occur via a "Southern Mode" in which the flow is strongly steered by topography and trapped close to the Antarctic continental slope. This mode is reflected in both sea level and ocean bottom pressure close to Antarctica. Here, we use altimetry and in-situ measurements together with a 50-year eddy-permitting ocean model run, to re-examine the role of the Southern Mode on longer time scales. We find that, when viewed in terms of bottom pressure, the Southern Mode continues to be an excellent representation of circumpolar transport changes on much longer time scales, but that bottom pressure and sea level start to behave differently at periods longer than about 3 years, with sea level showing an amplified response to transport changes at longer periods, as expected for changes which are equivalent-barotropic rather than barotropic. We consider how well transport could be monitored with a combination of GRACE and in-situ pressure measurements. Correlation of ocean bottom pressure and total flow through Drake Passage, for 50 years of 5-day means as simulated in the 0.25 degree NEMO model. Contours show depths of 1000 and 3000 m